Equations on a blackboard
An equation was a mathematical relation between a number of variables, which could be represented either by letters or numbers.
The astronomer Tycho Brahe from Earth lost his nose in a duel over a math equation. (ENT: "The Andorian Incident")
In 2367, Reginald Barclay and a hologram of Albert Einstein discussed a set of complex equations written on a blackboard. (TNG: "The Nth Degree")